1. Field of the Invention
The present invention relates generally to defibrillation processes, and more particularly to a realization that there exists an optimum capacitor value for defibrillation-pulse generation in an implantable system, a value smaller than recognized heretofore.
2. Description of the Prior Art
Defibrillation, or causing the cessation of chaotic and uncoordinated contraction of the ventricular myocardium by application of an electrical voltage and current, in its most primitive form goes back to the last century. [J. L. Prevost and F. Batelli, "Sur Quelques Effets des Deacharges Electriques sur le Couer des Mammifers, "Comptes Rendus Hebdomadaires des Seances de L'Acadmie des Sciences, Vol. 129, p. 1267, 1899.]
The sophistication and effectiveness of defibrillation techniques have grown rapidly in subsequent decades. One of the most recent developments has been the practical advent of implantable defibrillation systems. [R. A. Winkle, et al. "Long-term Outcome with the Implantable Cardioverter-Defibrillator," J. Am. Coll. Cardiol., Vol. 13, p. 1353, 1989; M. H. Lehman and S. Saksena, "Implantable Cardioverter-Defibrillators in Cadiovascular Practice: Report of the Policy Conference of the North American Society of Pacing and Electrophysiology," PACE, Vol. 14 p. 107, May 1990.] With the acceptance of this technology, the new challenge is to reduce system size while preserving its effectiveness, in order to improve the patient's quality of life and to extend the range of application such systems. [R. A. Winkle, "State of the Art of the AICD," PACE, Vol 14, p. 961, May 1991, part II; N. G. Tullo, S. Saksena, and R. B. Krol, "Technological Improvements in Future Implantable Defibrillators," CARDIO, Vol. 7, p. 197, May 1990.] Until an ability to anticipate fibrillation has been achieved, it will be necessary to achieve defibrillation by passing a large current through the heart. The current must be large enough to depolarize a large fraction of the myocardium, thus extinguishing depolarization wavefronts. [D. P. Zipes, et al., "Termination of Ventricular Fibrillation in Dogs by Depolarizing a Critical Amount of Myocardium," Am. J. Cardiol., Vol. 36, p. 37, July 1975.] Further, the waves must be strong enough so that the cells will not be stimulated during their vulnerable periods, causing refibrillation. [P. S. Chen, et al., "Comparison of the Defibrillation Threshold and the Upper Limit of Ventricular Vulnerability," Circulation, Vol 73, p. 102 May 1986.]
The high values of current that are usually employed in defibrillation procedures, and the compactness that is essential in implantable systems are conflicting requirements. For this reason, a huge premium is placed on knowledge of optimal values for various defibrillation-pulse characteristics; an optimum pulse will avoid the "waste" of current, charge, voltage, or energy, depending on which of these variables prove most relevant to successful defibrillation.
The components that dominate the physical volume of an implantable system are the capacitor and the battery, and here the avoidance of overdesign is crucial. A corollary to the proposition just stated is that accurate knowledge of which of the several defibrillation-pulse variables are dominant has an equally large premium placed upon it when an implantable defibrillator is to be designed. The present invention will address this challenge.
For reasons of simplicity and compactness, capacitor-discharge systems are almost universally used in defibrillation. Achieving the requisite electric field needed to depolarize most of the myocardial cells requires current density above a certain threshold value, and via Ohm's law, this means the process is favored by achieving sufficiently low electrical resistance in the discharge path. For this reason, the use of electrodes of relatively large surface area has for a long time been the norm. [A. C. Guyton and J. Satterfield, "Factors Concerned in Defibrillation of the Heart, Particularly through the Unopened Chest," Am. J. of Physiology, Vol. 167, p. 81, 1951.] The discharge of a capacitor C through a fixed resistance R results in a voltage-versus-time curve (and hence, current versus time as well) that is a declining exponential function, with a characteristic time given by the product RC. But, it has also been recognized for some time that the low-voltage (and low-current) "tail" of the capacitor-discharge pulse is detrimental. [J. C. Schuder, G. A. Rahmoeller, and H. Stoeckle, "Transthoracic Ventricular Defibrillation with Triangular and Trapezoidal Waveforms, " Circ. Res., Vol. 19, p. 689, October 1966; W. A. Tacker, et al., "Optimum Current Duration for Capacitor-discharge Defibrillation of Canine Ventricles," J. Applied Physiology, Vol. 27, p. 480, October, 1969.] The exact reason for this detrimental effect is not known, although plausible speculations exist.
Efforts to deliver a more nearly rectangular pulse over thirty years ago employed a series inductor in the discharge path, and improved results over the simple RC discharge were noted. [R. S. MacKay and S. E. Leeds, "Physiological Effects of Condenser Discharges with Application to Tissue Stimulation and Ventricular Defibrillation," J. Applied Physiology, Vol 6, p. 76, July 1953; W. B. Kouwenhoven and W. R. Milnor, "Treatment of Ventricular Fibrillation Using a Capacitor Discharge," J. Applied Physiology, Vol. 7, P. 253 November 1957.] Subsequent further efforts in the same direction used RLC (resistor-inductor-capacitor) delay lines, and reported further improvement. [R. C. Balagot, et al, "A Monopulse DC Defibrillator for Ventricular Defibrillation," J. Thoracic and Cardiovascular Surgery, Vol. 47, p. 487, April 1964.] But unfortunately, inductors are bulky components that are unattractive for incorporation in defibrillator systems, especially in implantable systems. For this reason, most efforts have been directed at time-truncated capacitor discharges. [J. C. Schuder, et al. "Transthoracic Ventricular Defibrillation in the Dog with Truncated and Untruncated Exponential Stimuli," IEEE Trans. Biom. Eng., Vol. BME-18, p. 410, November 1971.] That is, the capacitor discharge is simply interrupted by opening a switch at some middle point, typically, at about the time that the characteristic "RC time" has been reached, with a result approximating that of the example illustrated in FIG. 1. The advent of compact solid-state switches has made such pulse tailoring a straightforward matter.
The well-known waveform in FIG. 1 has for a long time been termed a monophasic pulse. It is generated by charging a capacitor from a high-voltage power supply which isolated from the capacitor by a rectifier, as illustrated in FIG. 2; then a switch in series between capacitor and heart is simply closed to initiate the pulse, and opened to terminate (or truncate) the monophasic pulse.
The amount of voltage decline (and current decline, assuming the heart to constitute a linear resistor) that has occurred at the time of pulse termination relative to the initial voltage, is termed the tilt of the pulse. In algebraic language, EQU tilt=(V.sub.initial -V.sub.final)/V.sub.initial. Eq. 1
Since the amplitude declines in one characteristic time to 1/e of its initial value, where e is the base of the Napierian system of logarithms, the tilt of a pulse terminated at the RC time is about 0.63, or 63%, to use the customary description. Typical values employed in monophasic defibrillation fall in the range from 60% to 70%. An example having a 67% tilt is shown in FIG. 3.
A waveform that is known equally well is the biphasic waveform, illustrated in FIG. 4. To create this kind of defibrillation pulse, the series switch is eliminated and a reversing switch that has long been used in the electronic art is employed instead. It consists of four switches in an "H" configuration, as illustrated in FIG. 5. Switches S1 and S4 are closed simultaneously to initiate the first phase of the waveform; to end the first phase these two switches are opened and simultaneously, the switches S2 and S3 are closed, creating a path for current to flow in the opposite direction from the capacitor through the heart. Opening all four switches terminates the second phase of the biphasic defibrillation shock.
The many studies that have been published on optional defibrillation-pulse properties have tended to focus on the energy stored in the capacitor, or charge times voltage. But with the truncation of the pulse, this total energy is unrelated to the energy delivered to the heart unless a number of other variable values are specified. Furthermore, in conventional systems, the residual charge stored in the capacitor is not recovered. Thus if one's aim, as at present, is to minimize the size and volume of an implantable defibrillation system, total stored energy is not a very useful criterion. Far more relevant is optimal capacitance. And given a particular value for the heart's resistance, with 50 ohms being a representative value for electrodes of typical design, then the amount of tilt is uniquely related to capacitance for a given pulse duration, and becomes an equally meaningful quantity. Finally, the pulse-duration optimum needs careful study.
Exhaustive scrutiny of published data in the literature shows that the prior art offers no determinations of optimal tilt or pulse duration for a capacitor of a particular size, nor does it offer any attempt to define an optimal value of capacitance for an implantable defibrillator. But the literature does offer a rich fund of data on defibrillation-pulse effectiveness presented in a way that permits a determination of the associated tilts and pulse durations used by those researchers. Furthermore, an extension of well-accepted physiological models results in a model that predicts optimal tilt and pulse duration for an arbitrary capacitance.
The present invention predicts the optimal capacitance for an implantable defibrillator.